It is known to use internal olefin sulfonates (IOS) as a surfactant for a variety of applications including chemical Enhanced Oil Recovery (cEOR).
Further, it is known to prepare olefin sulfonates from alpha-olefins and internal olefins, by sulfonating the olefins followed by neutralizing and hydrolyzing the sulfonated olefin containing intermediate mixture which comprises alkene sulfonic acids and sultones that are to be converted into the desired sulfonates. Said neutralization step and subsequent hydrolysis step both comprise contacting sulfonated olefin with a base containing solution, for example an aqueous NaOH containing solution. The hydrolysis step is generally carried out at a higher temperature than the temperature in the neutralization step, and is aimed at completing the reaction of the base with sulfonated olefin. See for example Adami, “The Production of α-Olefin Sulfonate by SO3 Sulfonation”, Section 5.3.1, pages 102-109, Handbook of Detergents, Part F: Production, CRC Press, 2009.
Further, EP0351928A1 discloses a process for the preparation of internal olefin sulfonates which comprises reacting in a film reactor an internal olefin having from 8 to 26 carbon atoms with a sulfonating agent, in a mol ratio of sulfonating agent to internal olefin of 1:1 to 1.25:1 while cooling the reactor with a cooling means having a temperature not exceeding 35° C., and allowing to neutralize and hydrolyze the reaction product from the sulfonation step.
More in particular, Example 14 of EP0351928A1 discloses a neutralization step wherein continuous neutralization of sulfonated C15-19 internal olefins from the preceding sulfonation step was performed at 30° C., at a residence time of about 20 minutes and at an active matter content of 26-31 wt. %. After the start of the continuous neutralization, a sample was taken after waiting for at least 1 hour and such sample was hydrolyzed for 1 hour at 160° C.
Said sample containing neutralized and hydrolyzed internal olefin sulfonate product from said Example 14 contained 6.7 wt. % of “free oil”, part of which (about 2 wt. %) was comprised of paraffins. In the Examples of EP0351928A1, the free oil content is calculated on the amount of active matter. Further, “free oil” is defined therein as “the petroleum ether extractable material in an aqueous-alcoholic solution of internal olefin sulfonates”. Therefore, in relation to internal olefin sulfonates, “free oil” may comprise any non-ionic, organic compounds that may be present in an internal olefin sulfonate product.
Such non-ionic compounds in the “free oil” in internal olefin sulfonate products may be unreacted internal olefins that have not been converted into alkene sulfonic acids or sultones. Further, such non-ionic compounds may be internal olefins formed by a back-reaction of sultones into internal olefins and SO3. Still further, such non-ionic compounds may be sultones that have not been converted into sulfonates. Further non-ionic compounds may be sulfonate esters and secondary alcohols formed from such sulfonate esters by saponification.
Additionally, an internal olefin sulfonate product may contain non-ionic compounds that cannot take part in the sulfonation and neutralization reactions. For example, (non-olefinic) paraffins present in the internal olefin feed cannot be sulfonated and therefore end up as non-ionic compounds in the internal olefin sulfonate product.
In general, it is desired to prepare an internal olefin sulfonate having a relatively low free oil content, more in particular having a relatively low content of internal olefins and/or sultones. For such relatively low content means that the sulfonation and neutralization reactions have completed to a greater extent, thereby wasting less valuable starting material and recovering more of the desired sulfonates. Besides, free oil represents a particularly objectionable impurity in the internal olefin sulfonate product, from the standpoint of its influence upon detergency, foaming, color, odor and other physical and chemical properties. There are methods known for removing free oil from final internal olefin sulfonate products after neutralization and hydrolysis. A particular method is for example disclosed in U.S. Pat. No. 4,579,690. However, such additional steps to remove free oil after neutralization and hydrolysis are cumbersome and time consuming. Therefore, it is desired that internal olefin sulfonates are prepared in such a way that the internal olefin sulfonate product itself already has a relatively low free oil content obviating the need for removal of free oil therefrom.
Therefore, the object of the present invention is to provide a process for preparing an internal olefin sulfonate wherein the obtained internal olefin sulfonate product has a relatively low free oil content, more in particular a relatively low content of internal olefins and/or sultones.